The invention relates to a display panel comprising a substrate having at least one compartment which contains an ionizable gas mixture, electrodes being present in the compartments for selectively ionizing the ionizable gas mixture during operation, said display panel being provided with an exhaust tube.
The invention also relates to a display device comprising such a display panel.
Display panels for displaying monochromatic images or color images comprise, inter alia, plasma-addressed liquid crystalline display panels, referred to as PALC displays, and plasma display panels (PDP). Such panels are used, for example, as displays for televisions and computer applications. In a plasma display panel, the plasma in the channels generates UV radiation which excites electroluminescent phosphors, and in a PALC display, the plasma is used to address a liquid crystalline material.
A display panel of the type described in the opening paragraph is known from, for example, the article xe2x80x9cHigh Resolution Displays and Projection Systemsxe2x80x9d by J. S. Moore et al. in the Proceedings of The International Society for Optical Engineering of Feb. 11-12 1992, California. The display panel of the flat-panel type described in this article is a PALC display and has a display screen with a pattern of (identical) data storage or display elements, and a number of compartments. The compartments are filled with an ionizable gas mixture and are provided with electrodes for ionizing the ionizable gas mixture. In the known display panel, the compartments have the shape of parallel, elongate channels provided in a substrate referred to as channel plate. The channels function as selection means for the display panel, referred to as the plasma-addressed row electrodes. By applying a voltage difference across the electrodes in one of the channels of the channel plate, electrons are emitted from the cathode. The electrons ionize the ionizable gas mixture so that a plasma is formed (plasma discharge). When the voltage across the electrodes in one channel is switched off and the gas is de-ionized, a subsequent channel is switched on. At the display screen side of the display panel, the compartments are sealed by means of a relatively thin dielectric layer, referred to as microsheet, which is provided with a layer of an electro-optical material, and further electrodes which function as the data electrodes or column electrodes of the display panel. The further electrodes are provided on a second substrate. The display panel is constituted by the assembly of the channel plate with the electrodes and the ionizable gas mixture, the dielectric layer, the layer of the electro-optical material and the further electrodes.
A plasma display panel (PDP) has an apertured selection structure provided with row and column electrodes in a vacuum envelope between a rear wall provided with a plasma cathode and a front wall provided with a display screen having pattern of luminescing pixels. In operation, a gas discharge is maintained between the plasma cathode and a series of row electrodes which function as anode. A number of the electrons from the gas discharge arriving at the anode passes the apertures of the selection structure at the location of a crossing of a row and a column electrode. The current through the selection structure is determined by the voltage applied across the column electrode corresponding to the relevant row electrode. As soon as the electrons have passed through the selection structure, they are accelerated in order to give the electrons sufficient energy to excite the luminescent pixels.
In both types of display panels described, the channel plate of the display panel is first vacuum-exhausted and subsequently filled with gas. Exhausting and filling are effected by means of, for example an exhaust tube. This exhaust tube may be directly fritted at the location of an aperture on the substrate surface remote from the display side of the display panel. Alternatively, an exhaust box to which the exhaust tube is connected may be present on the substrate at the location of said aperture. The exhaust box described in said article is made of a ceramic material and the exhaust tube is made of glass. After exhausting and filling with gas, the glass tube is sealed.
To extend the lifetime of such display panels, decrease the ignition voltage of the plasma and obtain a faster extinction of the plasma, the ionizable gas mixture in PALC displays preferably consists of an ionizable basic gas, for example helium, and an additional gas. The additional gas may be, for example, hydrogen, deuterium or hydrogen deuterium. To maintain the partial pressure of the additional gas substantially constant, additional gas is to be continuously added to the gas mixture. To this end, use is made of a getter in which the additional gas is stored. The getter may be secured by means of, for example a spring construction in the exhaust tube or, if present, in the exhaust box, the exhaust tube and the exhaust box communicating with the channel plate via an aperture in the surface remote from the display side of the display panel.
The exhaust tube described in said article is substantially perpendicular to the plane of the substrate. Since the exhaust tube is made of glass, it is very vulnerable. To this end, the exhaust tube should generally be made of glass which is sufficiently thick, which detrimentally influences the weight of the display panel.
It is an object of the present invention to provide a display panel in which the above-mentioned drawback is obviated.
To this end, the display panel according to the invention is characterized in that the exhaust tube extends in a plane parallel to that of the display panel on a substrate surface located opposite the display screen surface.
In this way, the exhaust tube is much less vulnerable than in a position substantially perpendicular to the plane of the panel. Besides, such a construction requires relatively little space so that the build-in depth of the display panel remains limited. It is sufficient to use an exhaust tube of relatively thin glass, which is to the benefit of the weight of the display panel. Moreover, the display panel can be laid on its rear side without any problem.
The exhaust tube may be directly fritted on the channel plate, but alternatively, an exhaust box may be present between the channel plate and the exhaust tube. The exhaust box described in the above-mentioned article is made of a ceramic material. However, ceramic material is rather heavy and also permeable to the ionizable gas mixture. To guarantee some impermeability, the walls should be fairly thick, which makes the exhaust box even heavier.
A preferred embodiment of the display panel according to the invention, in which the display panel comprises an exhaust box, is characterized in that the exhaust box and the exhaust tube are made of the same material as the substrate of the display panel.
Since both the exhaust box and the exhaust tube are made of the same material, it is not a necessity but they can be made as one assembly. Since the material of the exhaust box is also the same material as that of the substrate, the tension in the connection between the exhaust box and the substrate of the display panel will be reduced to a minimum. Moreover, an exhaust box of glass has a much lighter weight than an impermeable exhaust box of ceramic material.
If there is an exhaust box, the getter material may be provided in the exhaust box instead of in the exhaust tube.
A further embodiment of the display panel according to the invention is characterized in that the exhaust box is constituted by a widening of the exhaust tube, while at least a part is longitudinally ground open at the location of the widening.
As a result, the width of the edge of the exhaust box with which the connection to the substrate of the display panel is realized is thicker than the wall of the exhaust box itself. In this way, a sufficiently rigid fritting seam can be made by means of a thin-walled and hence light-weight exhaust box.
Moreover, the more components of the display panel are made of the same material, the simpler it will be to determine the loss of ionizable gas occurring during operation of the panel, so that relatively simple measuring methods can be used for this purpose.
A further embodiment of the display panel according to the invention is characterized in that the longitudinal axis of the exhaust tube and the longitudinal axis of the exhaust box enclose an angle between 30xc2x0 and 60xc2x0, preferably an angle of substantially 45xc2x0, the exhaust tube extending in a direction parallel to one side of the display panel.
This construction has the advantage that the glass exhaust tube is positioned just beyond the active part of the channel plate and will thus not be visible at the display side of the display panel.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.